sooo, on my MacBook (MacBook Air (M1, 2020) I clicked the F3 button (its like 3 rectangles) and once I was able to see both the desktop section and hearthstone section i dragged the hearthstone section into the desktop so i would exit fullscreen since I couldn’t click the actual exit fullscreen button.
Once it was dragged over to the desktop section it started working and I could see the game!!! Then, I exited the game, re-opened battle.net - logged in and when I opened hearthstone it worked even in fullscreen :)) i hope this helps my wording is tough to read lol